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Big quake to strike Sydney? Scientists wonder
Updated: 2004-07-20 02:43

Australia is shaken by about 200 tremours each year, but most register only about 3 on the Richter scale and usually occur in the outback, scaring a few birds and kangaroos.

But scientists say a major earthquake could strike Australia's largest city Sydney, population 4.5 million. Even a moderate quake could cause devastation and death.

The strongest quake on record in Australia registered about 6.9 on the Richter scale, matching the strength of Japan's Kobe earthquake in January 1995 that killed more than 6,400 people.

"Large earthquakes have occurred in sparsely populated areas. You can't discount that possibility, that a large earthquake might occur in the Sydney area,'' said Phil Cummins, head of the earthquake hazard team at the government agency Geoscience Australia.

In fact, every five years or so a potentially disastrous earthquake of 6.0 or more on the Richter scale rocks Australia and a quake as high as 7 is expected to occur every 100 years.

And scientists say the larger quakes are more likely to hit the edges of the Australian continent where the vast majority of the population lives.

Australia's most damaging earthquake, a moderate 5.6 on the Richter scale, occurred in December 1989. The quake struck the coastal coal-mining city of Newcastle, killing 13 people, injuring 130 and causing insured losses of nearly A$1 billion (US$733 million). Total estimated losses were A$4 billion (US$2.9 billion), including uninsured losses, damage to infrastructure and community disruption, according to Emergency Management Australia, a government disaster management body.

Newcastle, with a population of more than 300,000, is about 140 kilometres north of Sydney.

Earthquakes, brushfires, cyclones and floods have the potential to cause havoc in Australia and the largest exposure to most of these risks is Sydney, given its status as the country's centre of commerce and large population.

In the world of earthquakes, risk analysts play what they call "the catastrophe game" to figure out the odds of a quake hitting a city like Sydney.

There is only a small chance of a major earthquake striking Australia's eastern seaboard. But, as the Newcastle quake showed, they can happen and can cause extensive damage.

The frequency of earthquakes is lower in Australia than in its Asia-Pacific neighbours because the continent sits on the Indo-Australian tectonic plate, rather than on a volatile edge.

Earthquakes are most common where different tectonic plates meet, for example in earthquake-prone New Zealand to the southeast, which straddles the Indo-Australian and Pacific Plates.

Intraplate earthquakes, like those in Australia and continental North America, are less common and do not follow a pattern. They can also feel more violent, because intraplate earthquakes generally occur closer to the earth's surface.

When a large earthquake occurs within 10 km of the Earth's surface, the fault may rupture through the rocks that make up the Earth's surface and open up a long jagged rent.

Australia has had five such fault breakages in the past 30 years, more than any other country.

All five are in remote areas away from major population centres.

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